Interpretive Summary: Large weighing lysimeters are used by researchers to measure crop water use and develop models to accurately schedule crop irrigations. Two were installed in central California to determine water use in high value vegetable crops. As competition for water in the West increases, many growers will likely increase production of high value crops while reducing production of crops with less profit per acre in order to maximize income from limited water supplies. The results of the lysimeter work will help Central Valley farmers select irrigation systems and management strategies that have high potential for increasing profitability for growing crops in California and increase economic value per unit of water used. It may also reduce seasonal water requirements, allowing farmers to maintain yields even in the event of reduced water allocations or to use the saved water for production of other crops.

Technical Abstract:
Large weighing lysimeters are expensive but invaluable tools for measuring crop evapotranspiration and developing crop coefficients for horticultural crops. Crop coefficients are used by both growers and researchers to estimate crop water use and accurately schedule irrigations. Two lysimeters of this type were installed in central California to determine daily rates of crop and potential evapotranspiration and develop crop coefficients for better irrigation management of vegetable crops. Since 2003, the lysimeters were planted twice with broccoli, bell pepper, and head lettuce, and once with garlic. In each case, crop coefficients increased linearly or curve-linearly until canopy cover reached approximately 70%. Yield also increased linearly as more water was applied, but only when irrigation was scheduled as needed. Yield was much less affected by whether plants were irrigated by furrow or surface or buried drip. Crop coefficients reached a maximum value of 0.8 to 1.3, depending on the crop, but often declined prior to harvest. Developing accurate crop coefficients has the potential to increase yields significantly without increasing the amount of irrigation needed.